An electronic keyboard is extensively used as a user input interface in telephones, computers, and other electronic devices. A circuit coupled to the keyboard detects which key is pressed by a positive-logic scanning and a negative-logic scanning. For instance, as shown in FIG. 1, the keys of a typical keyboard are arranged matrix-like, where R1 to R4 corresponds to row 1 to row 4, and C1 to C4 corresponds to column 1 to column 4. When a key is pressed, its corresponding row line (e.g. R2) and column line (e.g. C3) are to be mutually connected. To attain that, in a negative-logic scanning circuit row lines are firstly set to high voltage level with high impedance, and column lines are set to low voltage level with low impedance. When a row line is connected to a column line, its voltage level drops to the low level. The states of the row lines are stored in a latch circuit. After that, column lines are set to the high voltage level with high impedance, and row lines are set to the low voltage level with low impedance. When the column line is connected to the row line as stated above, its voltage level drops to low level, and the states of the column line are stored in another latch circuit. By reading out the states stored in the latch circuits, the position of the pressed key can be easily determined. The steps described above are typically repeated for several times to confirm that the detected key has been intentionally pressed by user.
The keyboard shown in FIG. 1 is a typical 4.times.4 matrix keyboard. One of the integrated circuits (ICs) designed for scanning such a keyboard is the UMC91230 chip made by the United Microelectronics Corp. (UMC), which provides eight pins or ports for scanning, R1 to R4 and C1 to C4. Thus, the keyboard can only have 16 input keys. However, more keys are often required in modem applications, and number of pins for the keyboard scanning circuit IC increases as the number of keys which may be scanned increases. Increasing the number of pins raises the cost of manufacture of the IC.
In the detecting of keyboard functions in a telephone dialer, the functions are frequently tested by starting the oscillation of an oscillator after a telephone receiver is taken off-hook and enters into a detection mode. This detects whether there are any resistors grounded outside the keyboard input ports and therefore, determines the added functions of the dialer. The detection method has a drawback in that during the detection of any grounded resistors by the added function selection circuit, a false reading is sometimes obtained after detection caused by a poor cleaning process of the solder points on the circuit board which leads to noise interference during the detection. The process therefore is not effective in detecting the added function selection circuits of a dialer.
In a modem IC chip used in a telephone dialer, it is desirable to use an IC that has the least number of pins such that the package of the IC can be achieved inexpensively.
In the prior art detection method described above, the resistors used in the detection circuit frequently cause false readings due to the high impedance of the resistors used over the keyboard lines for more input function choices.
It is therefore an object of the present invention to provide a detection method for keyboard functions that does not have the drawbacks of the prior art detection method.
It is another object of the present invention to provide a method of detecting keyboard functions by using diodes in the circuit such that the total number of input pin requirement on the IC chip is reduced.
It is yet another object of the present invention to provide a detection method for keyboard functions that produces reliable detect result without false readings.